Xylene isomers are important industrial raw materials, which usually come from naphtha crackers and reformers as a mixture. The xylene isomers mixture contains meta-xylene (m-xylene, MX), ortho-xylene (o-xylene, OX), para-xylene (p-xylene, PX) and ethylbenzene (EB), wherein o-xylene can be used for synthesizing phthalic acid, m-xylene can be used for synthesizing isophthalic acid or can be converted to p-xylene which is an important raw material for manufacturing polyester fibers, and ethylbenzene can be used for producing styrene monomer. Because the xylene isomers have close boiling points, it is difficult to separate them by distillation. In the prior art, a cryogenic crystallization technique has been developed to separate the xylene isomers mixture comprising cooling the mixture until p-xylene being crystallized and separating the p-xylene crystals therefrom. This technique is relatively energy and capital intensive. Moreover, the recovery is limited to 73% of the feed due to the solid-liquid equilibrium.
In the present industrial practice the separation for xylene isomer mixture is usually achieved by the adsorption on zeolite adsorbents under liquid-phase or vapor-phase operation. Several patents dealing with this method are available, such as U.S. Pat. Nos. 3,943,183; 4,051,192; 4,326,091 and 4,439,535. With this method, the desorbent is generally required, and p-diethylbenzene, isopropylbenzene, propylbenzene, and toluene are the most employed desorbents. The desorbent used is recovered by distillation which is relatively energy intensive. A comprehensive introduction to this adsorptive separation can be found in the text book written by D. M. Ruthven, entitled "Principles of Adsorption and Adsorption Processes" John Wiley & Sons, New York (1984).
E. Sautacesaria et al. in their article entitled "Separation of Xylenes on Y Zeolites in the Vapor Phase. 1. Determination of the Adsorption Equilibrium Parameters and of the Kinetic Regime," Ind. Eng. Chem. Process Des. Dev. 24, 78-83 (1985) observed that separation could be improved by gas-phase operation and the quantity of desorbent used was relatively lower.
Chung-Sun Tan, the inventor of present invention, and Jeng-Leei Tsay in their article, entitled "Separation of Xylene Isomers on Silicalite in Supercritical and Gaseous Carbon Dioxide" Ind. Eng. Chem. Res., Vol. 29, 502-504, 1990, reported an experimental study of the separation of an equal amount of p- and m-xylenes on silicalite using carbon dioxide as the carrier. The results showed that the operations in the gaseous phase carbon dioxide offered a better separation efficiency over those at supercritical conditions. The effects of temperature, pressure and flow rate on the effectiveness of separation were also examined. It was found that, for a pulse of 1.0 cm.sup.3 of xylene isomers and 39.5 g of silicalite, the most appropriate operating conditions were temperature around 85.degree. C., pressure of 700 psia, and flow rate of 15.0 cm.sup.3 /min.
The object of present invention is to provide a process for separation of ethylbenzene or ethylbenzene/p-xylene from a xylene isomers mixture by selective adsorption on silicalite adsorbent, wherein a compressed gaseous CO.sub.2 is used as a carrier and a supercritical CO.sub.2 is used as a desorbent. As a result, the conventional desorbents are not used in the present process and thus the distillation separation for desorbents is prevented.
Another object of the present invention is to provide an integrated process for separation of ethylbenzene or ethylbenzene/p-xylene from a xylene isomers mixture by selective adsorption on silicalite adsorbent and recycling the CO.sub.2 used therein, wherein the CO.sub.2 is separated from the xylene products by an isothermal and isobaric selective adsorption separation and is recycled to the fresh CO.sub.2 feed without additional heating and compressing.